1. Field of the Invention
This invention relates to optical devices and graded refractive index lenses.
2. Discussion of the Related Art
A graded refractive index (GRIN) lens has a refractive index whose value varies with radial distance from the axis of the lens. The non-trivial variation in refractive index causes light refraction and gives the GRIN lens focusing capabilities that are similar to those of an ordinary lens. Therefore, many optical devices employ GRIN or ordinary lenses interchangeably.
Optical devices use lenses to focus, collimate, or expand light beams. FIG. 1 shows a fiber device 10 in which a GRIN fiber lens 11 is fused to a terminal end 12 of an optical fiber 13. The GRIN fiber lens 11 expands the light beam emitted by the optical fiber 13. The GRIN fiber lens 11 improves the optical coupling between optical fiber 13 and fiber device 15 as compared to the coupling that would otherwise exist between the fiber 13 and device 15 due to diffraction. The GRIN fiber lens 11 reduces diffraction losses when the optical fiber 13 is optically coupled to another optical fiber.
Since the diameter of a light beam varies along the axis of a GRIN lens, the diameter variations provide a measure of the lens' length. The length over which the variations in the beam diameter make two complete cycles is known as the pitch of the lens. Typically, lengths of GRIN lens are referred to in multiples of the pitch length, e.g., ½ pitch or ¼ pitch.